One thing that tends to work really well with cats is hissing at them. It is the noise they make themselves when they are startled or aggressive. The reason cats hate aerosols is because of the hissing sound they make. As soon as the cat bites, make a short, but loud, hissing sound, and move your hand abruptly. The cat will react. Do it every time he bites, and hopefully he will stop biting within a very short time.

Unlike dog aggression, feline biting for the most part is a playful activity. Cats seldom indulge in aggression without provocation, but they are likely to react aggressively if provoked too much. Physical distress is a factor that can provoke a cat to bite (whether related to the normal aging process or illness), so consult a veterinarian if it is suspected that the aggression is linked to discomfort. Cats are actually very predictable animals, and seldom do anything without a reason. Cats that bite or indulge in excessive scratching often develop the habit due to human negligence. Playful biting and scratching are integral parts of cat behavior, so an effort must be made on the part of the owner to establish boundaries. When a cat is not taught that human hands are not toys, they are bound to bite or scratch whenever they feel like playing. Since your cat is 4 years old, a natural remedy would probably be very helpful in addressing this behavior, since it may be a little late to establish proper training habits. -Aggression Formula™ promotes soothed behavior in dogs and cats(http://www.nativeremedies.com/petalive/aggression-formula-cat-dog-behavior.html) -PetCalm™ is an effective tonic for pet nervous system and harmony (http://www.nativeremedies.com/petalive/petcalm-nervous-dogs-cats.html) -Gumz-n-Teeth™(http://www.nativeremedies.com/petalive/gumz-n-teeth-prevent-pet-gingivitis.html) supports healthy gums and teeth in pets and may help your cat if his teeth are troubling him or causing pain.

I like both of the previous answers, but I just wanted to let you know that we call those 'love bites'. Apparently, your cat gets carried away with his love bites, but if he is being very loveable and not just playing, I think he thinks he is expressing his love. My kitty used to nibble my cheek when I asked her for kisses (I say 'used to' since I had to put her to sleep last year.) My, how I miss those 'love bite-kisses'!